A Superconducting Accelerating Structure for Particle Velocities from 0.12- to 0.23-C

A split-ring resonator has been designed for an optimum particle velocity ß = v/c = 0.16 and a frequency of 145.5 MHz. The ratio of peak surface electric field to effective accelerating field in the resonator has been reduced 20% from the value obtained in previously developed split-ring resonators. The improved design results from the use of ellipticallysectioned loading arms and drift-tubes which have been enlarged to reduce peak surface fields and also shaped to eliminate beam steering effects in the resonator. All fabrication problems presented by the more complex geometry have been solved and a prototype superconducting niobium resonator has been completed. An accelerating field of 3.3 MV/m at 4 watts rf input has been so far achieved, corresponding to an effective accelerating potential of 1.17 MV per resonator.